Functional Testing of Bone Morphogenetic Protein (BMP) Pathway Variants Identified on Whole-Exome Sequencing in a Patient with Delayed-Onset Fibrodysplasia Ossificans Progressiva (FOP) Using ACVR1R206H -Specific Human Cellular and Zebrafish Models

Current and emerging treatment modalities for fibrodysplasia ossificans progressiva

INTRODUCTION

Heterotopic ossification (HO), acquired or hereditary, is a diverse pathological condition defined by the production of extraskeletal bone in muscles, soft tissues, and connective tissues. Acquired HO is relatively prevalent and develops mostly in response to trauma, although its etiology is unknown. Genetic forms provide insight into the pathobiological mechanisms of this disorder. Fibrodysplasia ossificans progressiva (FOP) is a rare hereditary form of HO that can have a significant impact on affected individuals. FOP steadily weakens affected subjects and increases their risk of death.

AREAS COVERED

The U.S. Food and Drug Administration has recently approved the retinoid palovarotene as the first compound to treat heterotopic ossification in patients with FOP. This review provides a comprehensive overview of current and potential future pharmacotherapeutic options and their modes of action. The online databases PubMed, Cochrane Library, Web of Science, and ClinicalTrials.gov were searched using the terms 'heterotopic ossification' and 'fibrodysplasia ossificans progressiva' or synonyms, with a special focus over the last 5 years of publications.

EXPERT OPINION

Approval of palovarotene, as the first retinoid indicated for reduction in the volume of new HO, may revolutionize the therapeutic landscape. However, long-term safety and efficacy data for palovarotene are currently lacking.

Animal models of tendon calcification: Past, present, and future

Tendon calcification is a common clinical condition that frequently occurs as a complication after tendon injury and surgery, or as an expression of fibrodysplasia ossificans progressiva. This condition can be referred to by various names in clinical practice and literature, including tendon ossification, tendon mineralization, heterotopic ossification, and calcific tendonitis. The exact pathogenesis of tendon calcification remains uncertain, but current mainstream research suggests that calcification is mostly cell mediated. To further elucidate the pathogenesis of tendon calcification and to better simulate the overall process, selecting appropriate experimental animal models is important. Numerous animal models have been utilized in various clinical studies, each with its own set of advantages and limitations. In this review, we have discussed the advancements made in research on animal models of tendon calcification, with a focus on the selection of experimental animals, the sites of injury in these models, and the methods employed for modeling.

Matrix metalloproteinase-9 deficiency confers resilience in fibrodysplasia ossificans progressiva in a man and mice

Single case studies of extraordinary disease resilience may provide therapeutic insight into conditions for which no definitive treatments exist. An otherwise healthy 35-year-old man (patient-R) with the canonical pathogenic ACVR1R206H variant and the classic congenital great toe malformation of fibrodysplasia ossificans progressiva (FOP) had extreme paucity of post-natal heterotopic ossification (HO) and nearly normal mobility. We hypothesized that patient-R lacked a sufficient post-natal inflammatory trigger for HO. A plasma biomarker survey revealed a reduction in total matrix metalloproteinase-9 (MMP-9) compared to healthy controls and individuals with quiescent FOP. Whole exome sequencing identified compound heterozygous variants in MMP-9 (c.59C > T, p.A20V and c.493G > A, p.D165N). Structural analysis of the D165N variant predicted both decreased MMP-9 secretion and activity that were confirmed by enzyme-linked immunosorbent assay and gelatin zymography. Further, human proinflammatory M1-like macrophages expressing either MMP-9 variant produced significantly less Activin A, an obligate ligand for HO in FOP, compared to wildtype controls. Importantly, MMP-9 inhibition by genetic, biologic, or pharmacologic means in multiple FOP mouse models abrogated trauma-induced HO, sequestered Activin A in the extracellular matrix (ECM), and induced regeneration of injured skeletal muscle. Our data suggest that MMP-9 is a druggable node linking inflammation to HO, orchestrates an existential role in the pathogenesis of FOP, and illustrates that a single patient's clinical phenotype can reveal critical molecular mechanisms of disease that unveil novel treatment strategies.

Functional comparison of human ACVR1 and zebrafish Acvr1l FOP-associated variants in embryonic zebrafish

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by progressive heterotopic ossification of muscle and connective tissues, is caused by autosomal dominant activating mutations in the type I receptor, ACVR1/ALK2. The classic human FOP variant, ACVR1R206H , shows increased bone morphogenetic protein (BMP) signaling and activation by activins.

RESULTS

Here, we performed in vivo functional characterization of human ACVR1R206H and orthologous zebrafish Acvr1lR203H using early embryonic zebrafish dorsoventral patterning as a phenotypic readout for receptor activity. Our results showed that human ACVR1R206H and zebrafish Acvr1lR203H exhibit functional differences in early embryonic zebrafish, and that human ACVR1R206H retained its signaling activity in the absence of a ligand-binding domain (LBD). We also showed, for the first time, that zebrafish Acvr2ba/Acvr2bb receptors are required for human ACVR1R206H signaling in early embryonic zebrafish.

CONCLUSIONS

Together, these data provide new insight into ACVR1R206H signaling pathways that may facilitate the design of new and effective therapies for FOP patients.